I don't and no one should, there are a zillion threads out there with Eheims failing in the on position and cooking tanks. It's such an issue that even BRS suggests tossing your heaters once every year or two and getting new ones. At a minimum you should have a temp controller but I prefer on not having to rely on a system to shut something off. Using two slightly undersized heaters solves the same problem and involves 0 controllers to do it. The odds are astronomical that two heaters would fail in the on position at the same time in the same tank. And even at that it still wouldn't have the same effect as one oversized heater. Just my opinion, I won't ever run a heater that's rated for more than my tank...